The third generation partnership project (3GPP) and long-term evolution (LTE) standards are orthogonal frequency division multiplexing (OFDM) technologies. In addition, the LTE physical layer (PHY) uses orthogonal frequency divisional multiple access (OFDMA) on the downlink and single carrier frequency division multiple access (SC-FDMA) on the uplink.
OFDMA is used to multiplex different users with time-frequency resource allocation in the same transmission frame. Each user is allocated one or more sub-channels made up of a fixed number of narrowband subcarriers. Because they are orthogonal to one another, interference levels are generally low between subcarriers in a single cell/sector.
Despite the benefits of orthogonality, where the same subcarriers are in the same time slots within nearby cells/sectors, collisions (interference) may occur. Fractional frequency reuse (FFR) is one mechanism to address the interference. FFR distinguishes cell edge users from cell center users by providing cell edge users with more transmission bandwidth, more power, or both.
The transmission channel is divided into multiple subcarriers, which allows data to be transmitted in parallel streams. A downlink signal to be transmitted is schematically depicted using a two-dimensional resource block made up of the subcarriers (frequency) and OFDM symbols (time). An individual unit of the resource block is known as a resource element (RE). The resource element thus represents one subcarrier for sending data over one symbol period. The term physical resource block (PRB) is used in both the uplink and downlink, with RB being a shorthand version of PRB.
The resource elements can be assigned individually to different users in a cellular network. This allows data to be simultaneously sent to or received from the different users, subcarrier-by-subcarrier, for the designated symbol periods, by a single base station (eNB).
Under LTE, the resource block consists of twelve subcarriers per slot, where a slot is 0.5 milliseconds in duration, and two slots make up a subframe. Each resource element within the resource block grid represents a single subcarrier for one symbol period. For eNBs and user equipment having multiple antennas, or multiple-input-multiple-output (MIMO) applications, the transmitting antennas may send signals simultaneously within the resource block.
Before data transmission can take place between the eNB and user equipment (UE), the channel between them must be characterized. To characterize the channel, each eNB in the cellular network periodically sends synchronization signals and reference signals to the one or more UEs within its cell region. The synchronization signals provide network timing information, while the reference signals help to determine a channel impulse response. Another channel characterization process, known as “sounding”, involves the transmission of sounding packets to the user equipment.
Reference signals are embedded in the resource blocks in predetermined locations. For example, under the LTE specification, reference signals in the downlink are transmitted during the first and fifth OFDM symbols of each slot when a short cyclic prefix is used and during the first and fourth OFDM symbols when a long cyclic prefix is used. Reference symbols are also transmitted every sixth subcarrier and are staggered in both time and frequency. The reference signals are known to both the eNB and to the user equipment. A reference signal is assigned to each cell within a network and acts as a cell-specific identifier.
For the subcarriers (resource elements) bearing reference signals, the channel response is computed directly, while the channel response for the other subcarriers is interpolated. Where the eNB has multiple antennas, the reference signals may be transmitted sequentially from each antenna port that is formed by multiple antennas.
Two channels used in downlink LTE are the physical downlink shared channel (PDSCH) and the physical downlink control channel (PDCCH). The shared channel, PDSCH, transmits the data while the control channel, PDCCH, indicates user equipment-specific information. The PDCCH is mapped onto resource elements in up to the first three OFDM symbols in the first slot of the resource block.
For eNBs having two or more antennas, transmit beamforming may be used for improved throughput. Where the channel characteristics between the eNB and the user equipment are known, the phases of the transmit signals on each antenna may be coordinated so as to constructively combine at the user equipment antenna.
Under the LTE standard, the user equipment is configured in one of several different transmission modes, which defines how to process data transmissions received on the PDSCH. There are nine transmission modes under the most recent version of the standard, and the transmission mode is selected based on the capabilities of both the eNB and the user equipment.